1. Field of the Invention
This invention relates to engines and in particular to inferring the torque produced by an engine. One use of such an inferred engine torque is as an input parameter for a control system governing operation of an automatic transmission associated with the engine.
2. Prior Art
Knowledge of engine torque is required as an input for automatic transmission control strategies and transmission failure mode management strategies.
Direct measurement of engine torque is not currently practical so various inferences have been used over the years in both mechanically controlled powertrains and microprocessor controlled powertrains.
Known mechanical transmission control systems predicted torque from throttle angle or engine intake manifold vacuum and an aneriod atmospheric pressure sensor.
These mechanical systems suffered from many sources of error including the influence of exhaust gas recirculation rate on the prediction, changes in accessory torque losses, and changes in friction torque during the warmup period or over the life of the engine.
Known microprocessor based powertrain control systems inferred engine torque from engine speed and engine airflow. For systems equipped with a manifold pressure sensor the airflow was calculated after accounting for the influence of exhaust gas recirculation. Thus this source of error was minimized. A lookup function of air conditioning torque versus engine speed was used to adjust the predicted torque when the air conditioner clutch was enabled by the microprocessor.
These early microprocessor based torque prediction systems were subject to the effect of changes in spark advance, EGR percent, and air/fuel ratio which occur over various barometric pressures, ambient temperatures and time since engine startup.
Referring to FIG. 2, also known in the prior art is an electronic engine control module including a stored table 118 which is a MBT spark adjustment for air/fuel read only memory function and has an input from air/fuel ratio input 115. A block 119 is an indicated torque adjustment for air/fuel ratio and has an input from air/fuel ratio input 115. A block 120 has a stored table for an unadjusted MBT spark and has inputs from engine speed rpm input 116 and air charge input 117. A block 121 is an unadjusted indicated torque round table and has inputs from engine speed rpm input 116 and engine air charge input 117. A block 122 has a stored table indicating friction torque read only memory table and has inputs from air charge input 117 and engine speed rpm indicator input 116. A block 123 is a MBT spark adjustment for EGR, read only memory scalar and has an input from engine EGR rate input 114.
A summer 124 has positive inputs from block 118, block 123, and block 120 to provide an adjusted MBT spark which is the sum of the outputs of blocks 118, 123 and 120. A summer 125 has a negative input from spark timing input 113 and a positive input from adder 124 to determine the difference between MBT and actual spark advance. The output of summer 125 is applied to a table storing the indicated torque adjustment for spark retard in a block 126. The output of block 126 is applied to a multiplier 127 which also has inputs from block 119 and block 121. Multiplier 127 provides an adjusted indicated torque which is the product of blocks 119, 121, and 126. The output of multiplier 127 is applied to a summer 128. As an output of block 128, brake torque is indicated less friction torque. The output of summer 128 is applied to a coupling to transmission 111.
This level of microprocessor based control system had inaccuracies due to air conditioning head pressure, power steering torque requirements, cold engine friction, green engine friction, intentional or otherwise known cylinder shutdown (via spark or injector turnoff), and the influence of non-gasoline fuels like methanol/gasoline blends in flexible fuel vehicles.
A real time torque calculation is accomplished by determining the distance (i.e., actual spark advance) from MBT spark (SPK.sub.-- DELTA) by looking up MBT spark (FN1617) and adjusting it for EGR (MBTEGR) and LAMBSE (FN730) then subtracting the actual current spark (SAFTOT). Indicated (COMBUSTION) engine torque is then looked up from a table of RPM and LOAD (FN1615A) and multiplied by a function of SPK.sub.-- DELTA (FN621) and a function of LAMBSE (FN623).
Thus an improved torque prediction algorithm would be desirable to account for operating variables which influence torque output including engine coolant temperature and air conditioning friction.